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Home > BART > Hypoxia and Anoxia


Bay Line: 222-8888
(May 15 to Oct. 15)

Hypoxia (Low Oxygen) and
Anoxia (No Oxygen)

Oxygen is essential for life, but the supply in coastal waters around the world is decreasing - dropping more frequently, over larger spans of time and space. "Hypoxia" is a name for this condition when it turns critical - less than 2-3 milligrams of oxygen per liter of water. With "anoxia" there is none (0 mg/L oxygen).

The condition is critical because most sea life cannot tolerate it. Fish, crabs, shrimp, and shellfish that cannot flee become stressed or die with effects that ripple throughout the ecosystem. It becomes a "dead zone" that is hospitable only for anaerobic microbes that produce environmental problems of their own.

Many conditions contribute to hypoxia. Warm water (e.g., in the summer, in shallows, or near factory effluent) holds less oxygen than cold. Sheltered coves with weak winds and currents allow water to stratify, to form a persistent, low-oxygen band rather than to mix and refresh from top to bottom. In stagnant waters, hypoxia or anoxia is a natural condition, but it was rare in the Bay until humans significantly altered it.

Acute incidents are usually part of a chain reaction, beginning with an excess of nutrients in the watershed. Nitrogen comes from sewage treatment plant effluent and run-off of organic materials on fertilized grounds, or from bird and animal waste. Under certain conditions (usually as above) this over-enrichment feeds an algal bloom. The bloom creates shade, sinks, and then decomposes, drawing oxygen out of the water below. There is great demand but an inadequate fresh supply of oxygen. In the short term, the only cure is a change in the weather or the currents, enough to mix surface and subsurface, oxygenated and hypoxic waters. (See also "Fish Kills" and "Harmful Algal Blooms") The long-term solution is to moderate the supply of nutrients to the watershed.

What should I do?
  • Check for updates and advisories from Bay Line (222-8888, May 15 to Oct. 15) and HEALTH (See "Fish Kills" and "Swimming")
  • In the meantime, help prevent hypoxia by supporting efforts to improve water quality in the Bay. (See "Introduction to Narragansett Bay")
More information on-line

For General Information 222-6800 • After Hours Emergencies 222-3070 • Disclaimer
rev. 11/20/06